Fix for the issue of factory reset feature of things-manager not working.

Fix for the issue of factory reset feature of things-manager not working.

Things-manager service sends factory reset request to collection using
ActionSet structure. When user calls factory reset second time it was not
working as the ActionSet executed for first request had not been deleted.
Modified the thingsDiagnostic class for deleting the factoryReset
actionset everytime after its execution.

Change-Id: I312a0126a48ebd6dda6e5e887daed6f2a797de67
Signed-off-by: Harish Kumara Marappa <h.marappa@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/841
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Uze Choi <uzchoi@samsung.com>